This invention relates to the demodulation of differentially encoded quadriphase transmissions and more specifically to improved demodulation thereof.
In a communication system using quadriphase transmissions, information desired to be transmitted is impressed upon a carrier signal of a given frequency by effecting predetermined phase shifts in a carrier signal corresponding to the information to be transmitted. The transmission of this carrier signal is characterized as the differentially encoded quadriphase transmission which transmits four possible information words with each word consisting of two binary bits referred to hereinafter in this application as "dibits." Differentially encoded quadriphase transmission transmits each word as a given increment in phase relative to the absolute phase of the preceding transmitted word.
In utilizing demodulators for the above-cited differentially encoded quadriphase transmissions, the demodulator determines the phase increments of the received signal and decodes the binary data from these phase increments. These phase increments are detected utilizing a reference signal. The received signal and a 90 degree phase shifted receive signal are then multiplexed with the reference signal and a signal representing the reference signal shifted by 90 degrees. The resulting signals are fed into a plurality of integrators having an integration period equal to the period of one reference signal to obtain the detected I (in-phase) and Q (quadrature) signals from which the phase difference can be determined and proper coherent demodulation can then occur. Due to the quality of the transmission line or the like, the quality of the transmission signals deteriorate to the point that the recovery of the data transmission is very low. It is therefore an object of the present invention to provide a novel and improved data modulator apparatus. It is a further object of this invention to provide a data demodulator apparatus including a timing and synchronization circuit which corrects for any deterioration of the quality of the transmission signal.